2 posts from July 2015

07/30/2015

Antoine Bercovici is a Peter Buck postdoctoral fellow in the Department of Paleobiology at the National Museum of Natural History. He has been in the field since early July, and sent this post from North Dakota.

During the past two summers, members of the Department of Paleobiology conducted fieldwork in the badlands of North Dakota and Montana. They were collecting fossils for the Last American Dinosaurs Exhibit and contributing to decades of research aimed at unlocking the secrets of an ancient ecosystem. This ecosystem, known as the Hell Creek after the fossil-rich rock formation that formed here at the very end of the Cretaceous period, saw the iconic battle of the mighty Tyrannosaurus rex and Triceratops. But it was not just dinosaurs that lived there: many smaller animals thrived in this ecosystem, including crocodiles, turtles, lizards, salamanders, snakes, birds, fishes, and small mammals. They were living in a very lush world, comparable to today's Everglades, as indicated by the very diverse plant fossils also found in the rocks. This world came to an end 66.043 million years ago, as a giant asteroid hit the Earth in what is now the Yucatan Peninsula. The environmental disaster that followed caused many of the Hell Creek species to go extinct — most notably, the non-avian dinosaurs.

The North Dakota badlands show different layers of colored rocks. A major change in deposition can be seen here, indicated by the arrow: below are grey and monotonous sediments of the Hell Creek Formation, and above are the multi-colored sediments of the Fort Union Formation. Click to zoom.

This summer, our team is collecting new data that will tell us how stable the Hell Creek ecosystem was prior to the asteroid impact. We want to know how abundant the last dinosaurs inhabiting this part of North America were, and how their populations were changing over time. In order to answer these questions, we need to be able to place the fossils discovered here on an extremely accurate timeline. In geology, time is represented by the deposition of layers of sediments that accumulate on top of each other like a stack of pancakes: the deeper you go, the older the sediments and the fossils they contain. We are building our timeline by recording the elevations of Hell Creek fossil discoveries relative to the top of the Hell Creek Formation (the top of the pancake stack).

The mapping team is composed of Kirk Johnson, Sant Director of the National Museum of Natural History; Tyler Lyson, Curator of vertebrate Paleontology at the Denver Museum of Nature and Science; Gabi Rossetto, also from the Denver Museum of Nature and Science; Erica Evans of Colorado College; Dean Pearson of the Pioneer Trails Regional Museum in Bowman North Dakota; and myself. This mission involved a trip down memory lane as we pulled out old field notebooks to re-locate close to a thousand fossil sites that were discovered during the past thirty years of work in the Hell Creek of North Dakota. Walking back to every single site, we are marking their geographical positions with a Global Positioning System (GPS) unit and, more importantly, measuring their precise elevations.

Erica Evans (top), Tyler Lyson (right) and I point at a very distinctive white layer of sediment just above the Hell Creek - Fort Union formation contact. This layer marks the end of the Cretaceous and the demise of the non-avian dinosaurs; it is composed of material ejected from the crater that formed as an asteroid struck the Earth about 66 million years ago. We are using the GPS unit that Erica holds to measure the elevations of all our fossil sites relative to this layer. Click to zoom.

The GPS technology, developed and maintained by the US Army, is capable of incredible accuracy. In military use, it measures positions down to the nearest centimeter (cm). The system available to civilians, however, has a degraded resolution, with elevations typically measured only to within 10 meters (m), which is too much uncertainty to be useful for our project. (The Hell Creek Formation is about 100 m thick, so a 10 m error represents a 10% uncertainty in the position of a fossil site.) To get the accuracy needed for our project, we are using a technique called differential GPS, where we compare the readings of two GPS units to reduce the error. Every day, we set up a fixed GPS base station that records position and drift for several hours. We then map our sites using a mobile GPS unit that we carry with us. In the evening, the data recorded from both GPS units are downloaded to a computer, where dedicated software computes a correction, bringing the error at each site down to about 10 cm.

(Left) Erica Evans and Tyler Lyson setting up the GPS base station, nicknamed "The Brain,” early in the day. It will record the drift of the GPS position for a fixed point during several hours, allowing us to increase the accuracy of our field measurements. (Right) Erica Evans recording the position of a hadrosaur (duckbill dinosaur) rib that is eroding out of the hill. Click to zoom.

Our timeline fills with more and more fossil sites with each passing day of fieldwork. As we get a better idea of the distribution of our sites in time, we see gaps that we were not aware of before. Fortunately, the data also direct us to areas where we should prospect for new fossils that will fill the timeline gaps! Ultimately, we will have a very good understanding of how the fossils of dinosaurs and other animals are spread out in the 100 m thick Hell Creek Formation, and how ecosystems may have changed in the 1.3 last million years of the Cretaceous, before the asteroid impact.

So much ground to cover. Where should we go next? So far, we have visited about 300 fossil sites in an area of approximately 1000 square kilometers. Click to zoom.

Photos by Antoine Bercovici.

View earlier posts about Department of Paleobiology fieldwork in the Hell Creek Formation in 2013 and 2014.

07/17/2015

James Williams Gidley (1866-1931) was a vertebrate paleontologist at the United States National Museum (USNM), the forerunner of the National Museum of Natural History, during the early 20th Century. He was both a skilled fossil preparator and an active researcher, and his accomplishments in both areas are still felt within the Museum’s exhibits and collections. His career is described in a new essay by Department of Paleobiology volunteer Mark Lay, which was adapted for this post.

Dr. Gidley led or co-led approximately 20 field expeditions from the Museum to locations as far flung as Maryland, Montana, Indiana, Nebraska, Arizona, Oklahoma, and Florida. One of the most important was his collection and study of Pleistocene mammals from Cumberland, Maryland.

As the Western Maryland Railroad Company expanded its line west in 1912, workers exposed a small cave system while cutting through limestone rock on the side of Wills Mountain near Cumberland, Maryland. A local resident and amateur fossil collector, Raymond Armbruster, noticed that workmen were carrying fossil bones from the cave as curiosities, and sent a small sample to the USNM for examination. While caves are relatively common, caves containing fossils are not, so Dr. Gidley planned a trip to the site in October 1912. This initial trip resulted in the collection of more than 100 specimens, mostly jaws and jaw fragments from at least 29 different species of mammals, most of which are now extinct. This success was encouraging enough that Gidley and Armbruster collected sporadically at the site for another three years.

The Cumberland Cave in Maryland, circa 1913. The photo on the lefts shows the cave entrance, dwarfed by the size of the railroad cut. A tarp and wooden beams mark the opening, which is just left of center in the photo, at the level of the railroad tracks. In the photo on the right, James Gidley stands just outside the cave entrance. Photos by R. Armbruster. Click to zoom.

Although the limestone of Wills Mountain is of Devonian age, all the specimens collected from the cave and identified by Gidley are of Pleistocene age, suggesting that, roughly 200,000 years ago, the cave system was connected to the surface for a time by a sinkhole. Based on the probable topography, the sinkhole was approximately 100 feet deep, making it likely that anything that fell in would remain there. The fossils found are a mixture of animals that may have lived in the caves (such as bats, owls and invertebrates), some that may have fallen in, including terrestrial vertebrates (such as wolverines, bears, and peccaries), and numerous plants, all encased in sediments washed in over the years.

Several exhibit mounts were made from Gidley's Cumberland Cave fossils. The black bear and wolverine shown below were exhibited in the Ice Ages Hall until recently.

These mounts are relatively small in size, but Dr. Gidley’s overall impact on exhibits was huge – literally. He supervised the mounting of a whale, Basilosaurus cetoides, the “Indiana Mastodon,” and a large titanothere, Megacerops coloradensis.

Basilosaurus cetoides

Basilosaurus is a genus of Eocene whales that lived about 35-40 million years ago. Our specimen of Basilosaurus cetoides (USNM 4675) is a composite made from the skull and skeletal elements of three individuals collected by Charles Schuchert (Assistant Curator of Invertebrate Paleontology in the USNM) in Alabama in 1894 and 1896. At first, no attempt was made to reassemble the skeleton into a life-like pose, but after Frederic Lucas (Curator in the Division of Comparative Anatomy in the USNM) published a partial description of the whale’s anatomy in 1901 it became possible to consider creating a true restoration. Dr. Gidley was responsible for directing the creation of the mounted specimen, and the work was completed in 1912. This was the first mount of a Basilosaurus ever done. It measured in at approximately 55 feet long and was given a prime spot, shown below, in the Hall of Extinct Monsters.

Gidley's mount of Basilosaurus cetoides (USNM 4675) as it was first displayed in the Hall of Extinct Monsters. Photo courtesy of Smithsonian Archives. Click to zoom.

The mount survived essentially “as is” until 1989, when it was given some anatomical corrections and repairs and moved into the “Life in the Ancient Seas” exhibit (below, left). Then, in 2008, it was remounted in a new pose on a modern armature. The hind limbs were replaced with cast bones from another specimen, and the skeleton was moved to the Sant Ocean Hall (below, right), where it can be seen today. It remains the only real mounted Basilosaurus specimen on display in the world.

Photos by Chip Clark. Click to zoom.

Indiana Mastodon

Most of the American mastodons (Mammut americanum) found in this country are fragmentary. Complete (or relatively complete) skulls and skeletons are rare. One of those rare finds was reported to the USNM in 1914. On October 30th, George P. Merrill, Head Curator of Geology, wrote to the Assistant Secretary that “Mr. W. D. Pattison, a druggist of Winamac, Indiana, called at the office a few days ago and stated that in the work of ditching on some of his property in Indiana they had found a Mastodon skeleton, evidently in a good state of preservation; that they had removed only portions of the skull and one limb bone, which they would donate to us if we wished, and also give us the privilege of exhuming the entire skeleton.” The Museum paid for the bones to be shipped to Washington, where James Gidley examined them. Gidley reported to Merrill that, “the specimen… consist(s) of finely preserved portions of… an unusually large individual and with a minimum amount of preparation would make an attractive exhibit.” He went on to describe where a mount of such size could fit within the existing exhibit hall, concluding, “If, therefore, funds should become available at any time in the reasonably near future, I believe it would be well worth while accepting Mr. Pattison’s offer to exhume the remaining parts.” Funds must have been forthcoming because Gidley made two trips in 1915 to excavate and remove the rest of the specimen. The nearly complete skeleton (USNM 8204) required only limited restoration (done under Gidley’s direction) and the mount was put on display in 1916.

James Gidley stands with the newly mounted "Indiana Mastodon" (Mammut americanum, USNM 8204) in the Hall of Extinct Monsters. Click to zoom.

Megacerops coloradensis

A photograph (below) from our archives suggests that Gidley also supervised the creation of a third large display mount, that of Megacerops coloradensis (USNM 4262, this specimen had been excavated in Nebraska by John Bell Hatcher in 1887 and identified previously as Brontotherium hatcheri.) Dated 1919, the photo shows Gidley posed at the head of the mount with a sketch of the animal in his hands. Two men in work aprons, presumably Thomas Horne, who is known to have built the mount, and a helper, believed to be John Barrett, stand to either side of the skeleton. The Museums Annual Report of 1920 trumpeted both the skill involved in creating the mount and that, “this imposing addition [to the exhibits] … is the first and only mount to the genus to be exhibited.”

At work on the mount of Megacerops coloradensis (USNM 4262), a large Eocene herbivore. Click to zoom.

The Museum has always been justifiably proud of the many unique fossil specimens exhibited in its halls, and through the work of Gidley and others during the early decades of the 20th Century the number of these important mounts rose substantially. But with time they have become outdated. The mastodon and Megacerops, along with many other exhibit specimens, were disassembled recently as part of our ongoing National Fossil Hall renovations. After their bones have been conserved, they will be remounted on modern armatures and positioned in new stances that reflect 100 years of advancement in our scientific understanding of how these animals stood and moved. We have no doubt that James Gidley would approve of these changes to his century-old work.

Read more about the history of the Department of Paleobiology at the National Museum of Natural History, our collections and exhibits here.